1. Field of the Invention
This invention relates generally to percussion boring tools, and more particularly to percussion boring tools which are steerable and have means for reducing frictional forces during turning or arcuate movement of the tool.
2. Brief Description of the Prior Art
Utility companies often find it necessary to install or replace piping beneath different types of surfaces such as streets, driveways, railroad tracks, etc. To reduce costs and public inconvenience by eliminating unnecessary excavation and restoration, utilities sometimes use underground boring tools to install the new or replacement pipes.
While these tools are often effective, a significant problem of their operation is that their direction of travel cannot be controlled once they have penetrated into the earth. This lack of directional control decreases their usefulness because any deviations from the planned boring path cannot be corrected nor can the tool be steered so as to avoid obstacles or utilities in the established boring path.
Several steering systems have been developed in an attempt to alleviate this problem by providing control of the boring direction. However, experience indicates that the tool substantially resists sideward movement which seriously limits the steering response. A method is needed by which the tool can travel in a curved path without displacing a significant amount of soil inside the curve. Reducing this resistive side force would provide higher steering rates for the tools.
Therefore, the development of an economic, guided, horizontal boring tool would be useful to the utility industry, since it would significantly increase the use of boring tools by removing the limitations of poor accuracy and by reducing the occurrence of damage to in-place utilities. Use of such a tool instead of open-cut methods, particularly in developd areas, should result in the savings of millions of dollars annually in repair, landscape restoration and road resurfacing costs.
Conventional pneumatic and hydraulic percussion moles are designed to pierce and compact compressible soils for the installation of underground utilities without the necessity of digging large launching and retrieval pits, open cutting of pavement or reclamation of large areas of land. An internal striker or hammer reciprocates under the action of compressed air or hydraulic fluid to deliver high energy blows to the inner face of the body. These blows propel the tool through the soil to form an earthen casing within the soil that remains open to allow laying of cable or conduit. From early 1970 to 1972, Bell Laboratories, in Chester, N.J., conducted research aimed at developing a method of steering and tracking moles. A 4-inch Schramm Pneumagopher was fitted with two steering fins and three mutually orthogonal coils which were used in conjunction with a surface antenna to track the position of the tool. One of these fins was fixed and inclined from the tool's longitudinal axis while the other fin was rotatable.
Two boring modes could be obtained with this system by changing the position of the rotatable fin relative to the fixed fin. These were (1) a roll mode in which the tool was caused to rotate about its longitudinal centerline as it advanced into the soil and (2) a steering mode in which the tool was directed to bore in a curved path.
The roll mode was used for both straight boring and as a means for selectively positioning the angular orientation of the fins for subsequent changes in the bore path. Rotation of the tool was induced by bringing the rotatable fin into an anti-parallel alignment with the fixed fin. This positioning results in the generation of a force couple which initiates and maintains rotation.
The steering mode was actuated by locating the rotatable fin parallel to the fixed fin. As the tool penetrates the soil, the outer surfaces of the oncoming fins are brought into contact with the soil and a "slipping wedge" mechanism created. This motion caused the tool to veer in the same direction as the fins point when viewed from the back of the tool.
In underground percussion tools a method is needed whereby the torque required to rotate a percussion boring tool about its longitudinal axis while boring an underground hole can be reduced. This will significantly benefit the peformance of tool steering systems in which the tool is rotated to effect straight-hole operations and/or as a means of locating the steering device in a specific orientation.
The prior art has a significant number of steering systems. The tool steering system disclosed by Gagen & Jones in U.S. Pat. No. 3,794,128 uses one fixed fin and rotatable fin to steer the path of the tool. An important feature of this system is that the tool be able to rotate about its boring axis to effect straight boring and as a means to selectively orient the tool for steering. In this system the entire outer body of the tool contacts the soil wall. The use of torque reducing device which significantly reduces the contact area between the tool and the soil and provides a free spinning, lubricated bearing will greatly shorten the distance needed to effect a given rotation angle and will increase the overall steering response of this and similar steering systems.
Several percussion tool steering systems are revealed in the prior art. Coyne et al, U.S. Pat. No. 3,525,405 discloses a steering system which uses a beveled planar anvil that can be continuously rotated or rigidly locked into a given steering orientation through a clutch assembly. Chepurnoi et al, U.S. Pat. No. 3,952,813 discloses an off-axis or eccentric hammer steering system in which the striking position of the hammer is controlled by a transmission and motor assembly.
However, in spite of these and ther prior art systems, the practical realization of a technically and cost-effective steering system has been elusive because the prior systems require complex parts and extensive modifications to existing boring tools, or their steering response has been far too slow to avoid obstacles or significantly change the direction of the boring path within the borehole lengths typically used.
The prior art in general, and these patents in particular, do not disclose the present invention of a steerable percussion boring tool having means for reducing friction during boring and turning.